Metal Hydroxide Organic Frameworks (MHOFs) for Oxygen Evolution by Intercalation of Dicarboxylate Linkers Into β-Ni(OH)2: Impact of Host Crystallinity

Ni and Ni/Fe metal hydroxide organic frameworks (MHOFs) based on aromatic dicarboxylates are attracting great interest as emerging noble metal-free catalysts for the oxygen evolution reaction (OER), during H2 production by water splitting. They show good chemical and electrochemical stability during OER, some of them exhibiting similar catalytic activity compared to state-of-the-art catalysts (e.g., IrO2) when isolated as nanosheets. These MHOFs are actually precatalysts and reconstruct into the active NiOOH-like phases after leaching of the organic linker under electrocatalytic alkaline conditions. Employing β-Ni(OH)2 (herein indicated as NiOH-x) with different crystallinity (x = 1-low, 2-intermediate, and 3-high), we synthesized “bulk” forms of Ni2(OH)2(L) MHOFs (NiL-x) based on benzene-1,4-dicarboxylate (L = BDC2−) and azobenzene-4,4′-dicarboxylate (L = AZO2−) ligands. We systematically obtained highly crystalline NiBDC-x and NiAZO-x samples, regardless of NiOH-x crystallinity. Therefore, the intercalation of L2− effectively removes stacking faults disorder even from low crystalline NiOH-1, especially for BDC2−, while it is less efficient for AZO2−. The OER-activities were evaluated after establishing an activation procedure combining cyclic voltammetry with diffuse-reflectance UV–vis–NIR. NiAZO-x samples resulted the most stable and efficient catalysts, NiAZO-1 being the best among them although containing ∼50% in weight of residual NiOH-1, which converts superficially into highly efficient NiAZO-phase during synthesis, while retaining its high-conductivity in the inorganic bulk.